A cultured rat mast cell line, the RBL-2H3 cell, was shown to produce a TNFalpha- like protein upon stimulation with antigen or will the combination of phorbol myristate (PMA) and the Ca2+-ionophore, ionomycin. The protein was synthesized de novo. A significant proportion (approximately 50%) was released into the medium - a possible indication that the protein was incorporated into the secretory granules. The production of TNFalpha-like protein was inhibited with low concentration (1 nM) of dexamethasone, whereas exocytosis of granules and other early stimulatory events (hydrolysis of inositol phospholipids and increase in cytosolic Ca2+ levels) were inhibited only at high concentrations (100 nM) of steroids.. The findings may have important implications with respect to treatment of those allergic disorders in which a contributing factor is an inflammatory component that is associated with production of cytokines such as TNFalpha.